by Academician Valentin PARMON, Director of the G. K. Boreskov Institute of Catalysis, Siberian Branch, RAS

What with the scientific breakthroughs of the 20th century, one of the greatest puzzles of being - the origin of life on earth-is still there. A good many hypotheses have been advanced to this effect. Here's how Siberian scientists approach the problem.

According to the conventional, household view, the planets of the solar system are believed to have originated from the primary cloud of gas and dust which must have come into being about 5 billion years ago in consequence of some stellar cataclysms. But, as recent findings indicate, that cloud also doubled as a giant catalytic reactor containing iron, nickel and silicon (present in dust particles) as well as the key components of interplanetary gas-hydrogen and carbon monoxide... This family of chemical elements had to be implicated in the synthesis of organic compounds, a process triggered by cosmic particles. Our colleagues, upon a numerical modeling of astrophysical processes, have concluded that no planets could have come into being without such interactions. Solid particles of cosmic dust could not coagulate into larger concretions in the absence of "glue", the surface sticky layer of synthesized organic molecules. That layer caused the colliding dust particles of solid matter to conglomerate. Growing in size, such conglomerations gave rise to protoplanets from which the earth and other objects of the solar system were formed.

In fact, many of the essential hypothetical characteristics of the global cosmic reactor were known to chemists before. First, this reactor functioned by the same principles as its analogs with a catalyst's "pseudo-Hquefied layer". Second, the pressure of gaseous reagents in the zone of nascent planets must have been commensurate with the atmospheric pressure, i.e. quite typical of processes already known to specialists. Many other parameters, such as the composition of a reaction medium, temperature, etc., were similar as well. And thus the hydrogen and carbon monoxide on the surface of iron- and nickel-containing dust particles had to be involved, to begin with, in the synthesis of hydrocarbons and oxygen-containing organic matter like alcohols and aldehydes. We have obtained experimental proof that the substance of real meteorites does stimulate this process. Consequently, when putting forward any hypotheses on the geological and biological evolution of the planets, one should remember that planets are formed in the loci of the catalytic synthesis of organic compounds which, appearing at the incip-

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Cosmic catalytic reactor at work at the birth of the earth.

ient stage of the solar system, could have provided a basis for life. A portion of the organic matter could be captured by substances that subsequently became components of the present fossile deposits. Further studies should show whether this supposition is correct.

Now, what touched off physico-chemical processes that engendered life? To understand the events of that geologically short time, we should formulate our task as explicitly as possible. That is to say, we must agree on what in particular we understand by the phenomenon of "life" and what its first "prebiological" manifestation could be.

And yet a few essential attributes of life are well known. This is above all a process that involves an exchange of matter and energy with the environment. Next comes the reproduction stage, or self-replication. And last, the capacity of living objects for progressive evolution related to biological memory.

All these are but chemical processes. Taken apart, they are not life. For instance, the growth of crystals is nothing but the mechanical reproduction of similar compounds and structures. And these can be quite complex structures. Say, if we take a fine grid of aluminosilicate sieves, the ceolytes - in the elegance of their form and structure they can well measure up to the skeletons of tiny marine animals, the foraminifers of the protozoan order.

Our research institute is studying chemical processes based on exchange of matter (reagents and reaction products) with the surface of a catalyst. But this is not life either. Not at all.

Life, as we understand it, begins with biological memory that makes it possible to accumulate useful hereditary information thus preparing the ground for natural selection when organisms may get more complex in structure, adapt to their environment and proceed along the revolutionary path. Such memory is realized through the sophisticated molecules of RNA and DNA. Now, could there be any precursors of these molecules simpler in design but coping with their function nonetheless?

Dozens of theories propounded on our subject-matter break down into two different categories. One says life was born on the earth, period. While the other contends it was imported from outer space. Why this farfetched theory? None of the theories advanced so far explains the causes that led to the appearance of the very first molecules of RNA and DNA. The most cogent and well-argumented theory put forward by Oparin and Holdein in 1922 substantiates the formation of fairly complex chemical compounds and postulates the existence of a prebiotic broth composed of reactive organic matter from which biopolymers, and then fragments of individual organisms were synthesized. And so forth.

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Conception of life in the flows of energy and chemical substances according to Oparin and Holdein.

What this theory does not explain is this: how did the molecules of nucleotides come into being to give rise to RNA and DNA afterwards? They could not appear at random: even five billion years that our planet is believed to have existed would not be enough. But the view that such molecules sticking to cosmic dust could be brought in by comets or meteorites does not hold water since it fails to explain their formative mechanism.

Chemists can offer a fresh and unorthodox view of the problem by considering autocatalytic (self-catalytic) systems where chemical reactions are catalyzed by the end products of these reactions or of their precursors. Such autocatalytic systems are closest to biological ones. Here, too, molecules are capable of "multiplication", or self-replication; this simple reaction can be described by the equation R + X → 2X. A molecule of the self-catalyst X duplicates by reacting with a molecule R (substrate). If the substrate is large enough, the number of the autocatalyst molecules will proliferate in avalanche-like fashion.

Such reactions, should they proceed in an open system (characterized by metabolism, or exchange of substance, while the access to the "food", or substrate, is limited) conceal a paradox. This system can be in two steady-state conditions. In the first one the amount of an autocatalyst is always equal to zero, and its second molecule can come to be only if "primed", be it only by one precursor molecule. The other steady state sees a linear increase in the autocatalyst's amount with an increase in the concentration of the substrate. Yet this concentration should not exceed some critical, vital level, or else the self-catalyst will be "immobilized".

This characteristic is important. Should a system have several types of autocatalysts (self-catalysts), and not one, synthesized from the selfsame substrate, a fall in its concentration will kill autocatalysts - the first to go will be those subsisting on abundant amounts of "food". Such catalysts will never revive even if the substrate concentration is up again- there will be no "primer", be it one molecule only, to bring them back to life.

All that looks much like the natural selection in real populations and is quite on a par with some primitive biological memory: capable of "procreation" are only autocatalysts selected by the critical concentration of "food" parameter. Further events (mutations and resultant changes) will proceed from one and the same set of autocatalysts at which evolution came to a halt in its previous course.

Well and good, but do we know of just one autocatalytic reaction where mutations could occur? Yes we do. This is the Butlerov reaction discovered nearly 150 years ago by Alexander Butlerov, the great Russian

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"Natural" selection ofself - catalysts X, with an increase and subsequent decrease in the concentration of substrate R ("food"). During this cycle only self - catalysts X3 survives: the critical (vital) concentration of "food" Rcr3 proves to be lower for it than the concentration decrease (light dot).

Monosaccharides C6 in the Butierov reaction.

organic chemist and member of the St. Petersburg Academy of Sciences. In course of this reaction sugars are synthesized from the molecules of a very simple compound, formaldehyde (CH2 O). It can proceed even at room temperature and in water solutions in the presence of calcium or magnesium ions, i.e. components present in natural bodies of water. The Butierov reaction is still thought to be uncontrollable, for one cannot target it towards a synthesis of sugars of pre-assigned structure-every time it produces a set of most different sugars. This looks like a prototype of mutations. Should we make a study of several sugars of identical chemical composition, we shall find out that for all their identity in atomic composition, they exhibit different characteristics.

Now why is it so much important for sugars to be synthesized in the Butierov reaction? Because the two nucleir acids, RNA and DNA, carry the word "ribo" in their full name; and "ribo" is derived from "ribose", which is a sugar with five atoms of carbon, the backbone of any nucleotides.

RNA and DNA differ from the sugar molecules in that the sugar base of RNA and DNA nucleotides combines with the phosphate and nitrogen groups. The RNA and DNA nucleotides differ even less- they differ a bit in their nitrogen compounds, and in that the sugar residue of DNA contains fewer atoms of hydrogen.

Phosphorus and nitrogen compounds can bind to a sugar molecule in the course of simple spontaneous reactions. We might as well note here: the monosaccharide ribose is likewise predominant in another essential chemical component of living organisms, namely in the energy carrier ATP (adenosine triphosphate). That is sugars are the be-all of living matter. More than that, it is on their basis (and not on the basis of amino acids, or proteins) that molecules sustain the biological memory function and thus provide the divide between living and nonliving (inert) matter.

The Butierov reaction was studied a good deal in the 1970s in an attempt to develop a source of artifi-

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Monosaccharide ribose, the backbone of RNA and DNA nucleotides, and of ATP as well. The structures of homologous nucleotides urydyl (RNA) and thymine (DNA) shown.

Experiment in studying the Butlerov reaction under steady-state conditions.

rial food for long space flights to Mars. But all in vain: the mixture thus synthesized contained both useful and poisonous sugars. The root cause of the failure was that one did not solve self-catalysis problems.

The heart of the matter is this. Since we cannot control the course of an autocatalytic reaction as yet, we should begin by finding out what types of sugars act as the most vigorous self-catalysts. And then we shall learn whether natural selection is indeed possible in this chemical process. The role of calcium or magnesium ions is not clear either.

A few years ago our Institute of Catalysis (RAS Siberian Branch) resumed research into the Butlerov reaction. We are using rather simple "run-through" techniques enabling us to model many elements of systems "open" to exchange of matter. Several stock components are fed into a glass reactor where they are intermixed. A solution containing the end products flows out nonstop. Here's what is most important and most difficult to us: a chemical analysis of the substance obtained in the course of formaldehyde interconversions.

Even in our first experiments we discovered phenomena that had escaped our notice before. As it turned out, not all types of sugars are active as self-catalysts but only those in which one of the atoms of oxygen is located strictly at a definite point within a molecule. As shown by high-performance chromatography, the end products of the reaction proved to be absolutely identical in their composition and, under all other equal conditions, did not depend on the nature of a "primer" added in small amounts. We detected no less than ten synthetic sugars, with only three-glucose, sorbose and erythrose - well known. We are yet to learn what the other sugars are like. Biochemists say that the formation of nucleotides, the primary ones especially, did not necessarily implicate ribose-other sugars could give the go-ahead as well. As shown by experiments carried out by us, the simplest molecules of sugars can be obtained by irradiating aqueous solutions of formaldehyde with ultraviolet, i.e. without the Butler reaction.

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As to the "food" source (substrate) for sugars, formaldehyde: it was certainly present in the atmosphere of the protoearth, apparently in large amounts; it could be formed by powerful lightning discharges or on hot catalytically active lavas.

Today we can make certain tentative conclusions. First, the progressive natural selection on earth could begin in the absence of RNA and DNA and involve simpler compounds. Second, no prebiotic "thick" broth (i.e. rich in organic matter) was necessarily needed for the inception of life - we don't think that such kind of broth could ever be. According to our analysis, natural selection had its start in a "thin" broth in which the molecules of autocatalysts were competing for the "food" substrate. Another important consideration: thermodynamically, the primary synthesis of sugar molecules was more advantageous than the synthesis of proteins from amino acids commonly believed to be the primordial element of life. Why? In the synthesis of sugars from formaldehyde water molecules are not "knocked off' into an aqueous solution as it happens when proteins are synthesized from amino acids. Unlike protein molecules, those of sugars are stable even in much diluted water solutions.

The frequency of "useful" and "fixed mutations" of autocatalyst sugars is likewise important. They seldom occur in the populations of living organisms, and still less are fixed. Natural selection is a sluggish, very slow process. Under our experimental conditions both useful and detrimental mutations proceed within a few minutes. Further studies must indicate the real time of these processes under concrete conditions. Be that as it may, the first prototypes of living objects could appear within a few million and perhaps hundreds of thousands of years-not billions of years! Geological evidence attests to that.

The hypothesis on the role of the self-catalytic synthesis of sugars may help solve the problem of chirality (handedness) in nature, that is the existence of "right-" and "left-handed" isomers of molecules, as it is with gloves for right and left hands. It is no easy to explain why only "right-hand" gloves occur in present-day organisms. Assuming that life appeared as a result of selection in autocatalytic systems, it will be logical to infer that the presence of the "right glove" only is a random factor. In fact, the molecules of nearly all sugars are characterized by chirality. It could be that the very first molecule of an autocatalyst sugar - a molecule that had a significant evolutionary edge over the other molecules-made a short shrift of the rest. So nature had no other way out but use this primordial molecule with its random "right-handedness" for the construction of all subsequent and more complex molecules, and living organisms, too.

Accordingly, we can now give a fairly broad physicochemical interpretation of the concept "life". This is very important, for searching for its primitive forms elsewhere in extraterrestrial space, we may bypass certain phenomena that baffle observation. So, proceeding from what we have said above, life is a phasically isolated form of functioning autocatalysts (self-catalysts) capable of chemical mutations and having undergone a sufficiently long evolution through natural selection. Obviously, our interpretation is far broader than the notion of life predicated on DNA and RNA molecules only.

So far we have not touched upon the possibility of the phasically isolated forms of functioning autocatalysts. We can only hope that subsequent studies into the Butlerov reaction will elucidate this matter too. But the wide occurrence of phasically isolated catalytic systems is evident even today. For instance, in the course of the synthesis of such widespread polymers as polypropylene or polyethylene the end product is phasically isolated in the shape of a microgranule or globule. Within such microgranules or globules joined from a polymer's macro-molecules there is a catalyst at work, and it accumulates the synthesized material.

Yet another, though not quite obvious, conclusion follows from our analysis: if the Butlerov reaction is possible on the surface of this or that planet, the birth of life on the basis of RNA and DNA is predetermined. Which means that the other forms described by sci-fi authors (silicone, fluorine) just cannot be. RNA and DNA are bound to appear on any other planet with a geological history similar to the earth's.

It could be that the Butlerov reaction is not the only autocatalytic reaction underlying natural selection. But we know nothing about other ones as get. We must search for them by relying on fundamental studies of reactions in which organic molecules are synthesized. Thus we can detect the primary chemical processes antecedent to the inception of life.

To conclude, the author wishes to offer his thanks to the staff of our institute and to colleagues of the Biophysics Institute, and of the other physics and mathematics institutes of the RAS Siberian Branch whose works have been of much help for this article. The author owes special thanks to Novosibirsk State University where many chemical experiments are carried out with the hands of enthusiastic graduate students.

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